We have completed our participation in a randomized, multicenter, double blind, placebo-controlled phase III study to evaluate the effect of valaciclovir in preventing the transmission of HSV in heterosexual couples. The seropositive source partner was randomized to receive valaciclovir 500 mg once daily or placebo for 8 months, and the susceptible seronegative partner was monitored monthly for clinical and subclinical (serological) acquisition of HSV. Analysis of the study showed that once-daily suppressive therapy with valaciclovir 500 mg reduced transmission of symptomatic genital herpes by 75% versus placebo (0.5 percent vs 2.2 percent). In addition, suppressive therapy with valaciclovir reduced the overall acquisition of the virus by 48% versus placebo (1.9 percent vs. 3.6 percent). There were no differences in the adverse events profile between the two groups. In May 14 2003, the U.S. Food and Drug Administration Advisory Committee unanimously recommended the approval of once-daily suppressive therapy with valaciclovir for the reduction of transmission of genital herpes in otherwise healthy, heterosexual, monogamous couples. A manuscript was published at the New England Journal of Medicine in January 2004 reporting the results. We are currently discussing possible new vaccine candidates for therapy of herpes simplex, and planning new clinical trials.